Using monoclonal antibodies against mesenchyme cells (MCs) and a fibrous component (FC) of the extracellular matrix (ECM) we have studied interactions between the two during morophogenesis of the starfish, Asterina pectinifera. The FC was found to function as an endoskeleton of the embryonic body during the gastrula stage. Specifically, this function includes being a substratum for MC migration, that for endodermal sheet migration and constriction force for the posterior portion of the ectoderm. During invagination of the presumptive stomodeum, the MCs caused the FC to aggregate, which, in turn, generated the supporting force to the invagination process. MCs also repaired the structure of the FC when it was artificially disordered. The interaction of the MCs and FC was likewise observed in coelomic pouch formation in the reconstructing embryos, in which dissociated gastrular cells reaggregated and organized themselves into the body of the bipinnaria larva. During the bipinnaria stage, the MCs formed a network underlying the larval body wall and functioned as defensive cells. Together, we have concluded that MCs not only play an important roll in starfish morhogenesis through interaction with the FC, but also function as defensive cells in the larval stage.We have attempted to uncover molecules specific to the MCs and the FC.In the FC, a molecule (300 KDa) containing a carbohydrate domain was found to associate with other types of ECM molecules. In the MCs, a molecule (73 KDa) was shown to be proteinacious and localize to the plasma and to cytoplasmic membranes. We intend to analyze further the two molecules, in the future, to elucidate the molecular mechanism which underlie the interactions between the MCs and FC.